Electron Paramagnetic Resonance (EPR) provides a sensitive means of detecting and quantitating free radical species. Presently, its use for in vivo studies has not been feasible by the frequency used, routinely greater 1 GHz, where penetration of the radiation in tissue is minimal, with commercially available spectrometers. Because ionizing radiation ultimately results in formation of free radical species, the RBB has been developing a pulsed Fourier Transform (FT) EPR system for spectroscopy and imaging of larger biologic objects such as tissue, tumors and even small animals. Since magnetization of electrons relaxes in microseconds to nanosecond, the use of nanosecond FT-EPR with rapid signal sampling and averaging is required for detection of transient, short-lived free radical signals, particularly at Radiofrequencies (RF). Likewise, the previous technologic limitations imposed by CW-EPR may be overcome with FT-EPR when efficient data sampling/averaging are employed. The goals of this project are to develop a prototypical RF FT-EPR instrument for in vitro biologic studies and establish the foundation of RF FT-EPR in vivo imaging. The feasibility of this experiment at RF and l- and 2-dimension images of free radicals has been demonstrated. Fast responding electronic gates and amplifiers have been developed and integrated into the spectrometer. Resonators have been designed and optimized for spectroscopic and imaging experiments. An efficient sampler and averager have been designed and tested which demonstrates that the signal intensity of the free radical species can be enhanced to significant extent by acquiring the signal and averaging in short time intervals (<1s). Feasibility of this strategy in free radical detection and in vivo oxymetry of murine models has been demonstrated. Electronics and computation have been optimized to establish the minimum detectable quantities of free radicals. Such information will lead to the resolution in imaging and also the levels of the free radical probe to be utilized in spectroscopic and imaging experiments. With this information, imaging and oxymetry in larger animals will be attempted with practically useful free radical probes. Such an imaging device will have use in diagnostic applications such as in local sites of inflammation and malignancies which are small and below the resolution of other diagnostic methods.